1. Field of the Invention
The present invention relates to a beam source for fast atomic beams which are generated from plasma, wherein output energy can be controlled over a wide range of energy levels.
2. Description of the Related Art
Conventional fast atomic beam sources will be reviewed first. Atoms and molecules undergoing thermal motion in ordinary atmosphere have kinetic energy of generally about 0.05 eV. Compared to such relatively low kinetic energies, atoms and molecules moving with a much higher level of kinetic energy are referred to as fast atoms. When such fast atoms are radiated as a narrow directed beam, they are referred to as a fast atomic beam (shortened to FAB hereinbelow).
Of the various known sources for generating FAB based on gaseous atoms, FIG. 1 shows a schematic representation of an example of an argon-based FAB having a kinetic energy in a range of 0.5 to 10 KeV. The main components are a cylindrical negative electrode 1, a donut shaped positive electrode 2, a high voltage source 3, a gas inlet tube 4, an argon gas plasma 6, FAB emission holes 7 and FAB 8.
All components excepting the electrical power source for the beam source and a discharge stabilizing resistor (not shown) are housed in a vacuum container, and after the container has been sufficiently evacuated, argon gas is introduced into the cylindrical negative electrode 1 through the gas inlet tube 4. A direct current (dc) voltage is applied from high voltage dc source 3 so that the positive electrode 2 will be at a positive potential and the negative electrode 1 will be at a negative potential. The result is that an electrical discharge occurring between the positive and negative electrodes generates a plasma 6, thereby providing argon ions and electrons.
The electrons emitted from the bottom surface 1a of the cylindrical negative electrode 1 are directed to the positive electrode 2 and accelerated by positive potential, and after passing through the center hole in the positive electrode 2 reach the bottom surface at the opposite end of the cylindrical negative electrode 1, where the electrons lose speed and reverse their flight direction because of negative potential. The electrons, which are now moving in the opposite direction, begin to be accelerated towards the positive electrode 2. Thus, the electrons undergo high frequency oscillations in the plasma space between the bottom surfaces of the negative electrode 1 through the center hole in the positive electrode 2. The oscillating electrons collide with the argon atoms and generate many more argon ions. The argon ions thus generated are accelerated towards the bottom surface 1a of the cylindrical negative electrode 1 thereby attaining a kinetic energy. The level of kinetic energy is about 1 KeV, for example, when the potential difference between the positive and negative electrodes is 1 KV. The space near the bottom surface 1a of the negative electrode 1 is a U-shaped region for the high frequency oscillating electrons, and contains a large percentage of low-energy electrons. The argon ions which are injected into this space return to argon atoms by colliding/recombining with the electrons. Because the mass of electrons is negligibly small in comparison to argon ions, the kinetic energy of argon ions is almost not affected by the collision process with the electrons, and the kinetic energy of argon ions is transferred substantially to the argon atoms to produce a fast atomic beam. Therefore, the kinetic energy of the fast moving argon atoms is about 1 KeV. Fast moving argon atoms are emitted as an argon FAB from the discharge holes 7 which are provided in a bottom surface of the negative cylindrical electrode.
However, conventional FAB sources are suitable for applications which require a potential difference of higher than 1 KeV to generate atomic particles having sufficiently high discharge current to carry out processes thereof. At low voltages, it is only possible to generate a low discharge current beam, and therefore, it is necessary to adopt other means to obtain high beam density. The situation is the same when magnets are used to generate magnetic fields, and is characteristic of a dc discharge process. Therefore, low discharge current means that the volume of ions generated is low, and consequently, the strength of the output FAB beam is also low. Furthermore, the conventional FAB sources can only produce beams having poor directionality because of their high beam dispersion characteristics, which is unsuitable for satisfying critical requirements of modern micro-fabrication, wherein a FAB must be able to fabricate three-dimensional fine objects of high aspect ratios in any orientation.
Therefore, there has been a need for a source of generating fast atomic beams which is capable of producing high beam density, precision directionality and a wide range of output energy levels.